fix(deps): update kotlin to 2.3.0 (#5917)

* fix(deps): update kotlin to 2.3.0

* Cleanup - remove `datetime` compat version

* Fix several lint issues caused by the Kotlin compiler inference working better (checks in nullables, vars, etc.)

* Fix tests by removing mock in `File.readBytes`, it seems like it's no longer allowed. Using a tmp file works well enough.

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Benoit Marty <benoit@matrix.org>
Co-authored-by: Jorge Martín <jorgem@element.io>

gradle/libs.versions.toml

@@ -5,9 +5,9 @@
 # Project
 android_gradle_plugin = "8.13.2"
 # When updateing this, please also update the version in the file ./idea/kotlinc.xml
-kotlin = "2.2.20"
+kotlin = "2.3.0"
 kotlinpoet = "2.2.0"
-ksp = "2.2.20-2.0.4"
+ksp = "2.3.4"
 firebaseAppDistribution = "5.2.0"
 
 # AndroidX
@@ -62,7 +62,7 @@ detekt = "1.23.8"
 # See https://github.com/pinterest/ktlint/releases/
 ktlint = "1.8.0"
 androidx-test-ext-junit = "1.3.0"
-kover = "0.9.2"
+kover = "0.9.4"
 
 [libraries]
 # Project